The present invention relates to a system and method for probing the subsurface of the earth using electric currents. More particularly, the invention relates to the discrimination of features at depth within the earth from features close to the surface via an electromagnetic source that injects electrical current into the earth via different selectable sets of electrodes, one of which couples to a significant depth.
The embodiments described herein relate generally to soundings within the earth based upon electrical fields. As used herein, “earth” or “Earth” generally refers to any region in which a borehole may be located including, for example, the lithosphere. Electromagnetic (EM) geophysical surveys probe electrical resistivity, or equivalently, conductivity, in the earth as a function of depth. Typical targets of interest include ore bodies, hydrocarbons, water, proppants, hydraulic fracture (or fracking) fluids, salts and other substances injected into the ground, and environmental pollutants. Since the resistivities of such targets and the surrounding medium may be quite dissimilar, the targets may be discriminated by measuring their subsurface resistivities when subjected to an electromagnetic field. Using this methodology, the depth, thickness, and lateral extent of materials of interest may be determined or monitored.
The source of the EM field used in a geophysical survey may originate in the natural environment or be manmade. If manmade, the source may produce a primarily magnetic or electric field that varies in time, and this primary field produces a secondary field in the conducting earth. For example, an electric field produces electric currents in the earth that have an associated magnetic field, and a time varying magnetic field induces electric currents that result in an electric field. The electrical properties of the earth and rate of change of the field determine the relative magnitudes of the secondary and primary fields. The combination of primary and secondary fields results in combined electromagnetic interaction with the earth even for a source arranged to produce solely an electrical or magnetic field.
While the majority of EM geophysical surveys are performed with sensors and EM sources on the surface of the earth, a borehole can provide physical access to the subsurface. Measurement of the electric or magnetic field within a borehole can be related to the electric or magnetic field in the earth around the borehole or the fields that would exist in the earth in the absence of the borehole. Similarly, connecting an electric field or magnetic field source to the earth via a borehole provides a way to produce fields within the earth at desired depths without the attenuation and uncertainties that may result if the source fields originate from a source at the surface of the earth. A particularly beneficial configuration of a borehole EM source is an electrode situated at the top or bottom of a borehole casing, and in electrical contact with that casing, and a group or suite of source electrodes at the surface approximately arranged in a ring centered on the borehole. In this case, significant electric currents in the ground are caused to flow at depth out to a radial distance from the borehole to the surface electrode ring.
The distribution of electric current flow produced by an EM source is determined by the three-dimensional (3-D) resistivity distribution within the earth. The electric field measured at the surface or at depth within a borehole can be used to infer the 3-D resistivity variation over the region where significant current is flowing. The current is typically measured by a suitably calibrated array of electric or magnetic field sensors. The resulting 3-D resistivity variation can be used to project the distribution of ores, hydrocarbons, or water within the measured volume.
A common problem in applying this method of subsurface EM imaging is discriminating spatial changes in resistivity at the depth of the formation from those near to the location of the sensors. Recent models of the current flow from a ring of surface electrodes to a borehole casing show that current flows from the earth into the casing along the entire length of the casing. Thus, significant current flows near the surface of the earth between the source electrodes and the wellhead. This near-surface current causes a significant interfering measurement artifact for measurement configurations wherein receivers are located at the surface of the earth. The problem is that a resistivity anomaly (i.e., a local change in the earth's resistivity) near the surface causes a much larger change in the EM field at the surface than an equivalent resistivity change much deeper in the earth. Surface resistivity anomalies can be static or can vary over time if they are affected by weather events, such as rainfall, and temperature variations, such as freezing. Furthermore, surface anomalies, such as those produced by pipes and other electrically conducting infrastructure, can extend widely over the surface region and be difficult to separate from the signals produced at depth.
Therefore, there exists a need in the art for a system and method to separate, or predominantly separate, a signal of interest produced by a subsurface feature at depth within the earth from a near surface anomaly. Preferably, the method should not attenuate or otherwise reduce the spatial range of the EM survey.